Compared with a SISO (Single-input Single-output, single-input single-output) system, a MIMO (Multi-input Multi-output, multiple-input multiple-output) system may increase channel capacity. In a wireless MIMO system, it is assumed that a channel is a Rayleigh fading channel, that is, the channel has no direct path but has various multipaths. In this situation, capacity of a MIMO channel may increase linearly according to the number of antennas.
For a microwave channel, a system generally provides a relatively strong direct path, and in this situation, the channel can be simulated into a Rician fading channel. In the Rician fading channel, a channel matrix is typically in low-rank state. That is, a part of eigenvalues λi of a MIMO channel matrix is 0. According to the Shannon formula
      C    =          B      ⁢                        ∑                      i            =            1                    r                ⁢                              log            2                    ⁡                      (                          1              +                                                λ                  i                                ⁢                                                      P                    T                                    /                                      (                                                                  σ                        2                                            ⁢                                              n                        T                                                              )                                                                        )                                ,capacity of a MIMO system in low-rank state cannot be fully utilized, where λi is an eigenvalue of a MIMO channel matrix, PT is average signal power, σ2 is noise power, nT is the number of transmit antennas, r is the number of receive antennas, B is signal bandwidth, and C is channel capacity. A condition number in the MIMO system is defined as the ratio of the maximum eigenvalue of the channel matrix to the minimum eigenvalue of the channel matrix. Larger condition number results in stronger channel singularity. Smaller condition number approximating to 1 results in better orthogonality of the MIMO system. For a MIMO system with a Rician fading channel, when an antenna interval is a Rayleigh distance, multiple sub-channels of the MIMO system can maintain orthogonality, and a condition number of a channel approximates to 1, thereby supporting independent transmission of multiple data streams.
At present, two solutions are available for implementing microwave multiple-input multiple-output;
The first solution is as follows; Add a relay base station between microwave base station 1 and microwave base station 2, and generate independent transmission paths through forwarding by the relay base station, thereby increasing channel capacity. The essence of this solution is adding an independent sending path so that channel correlation approximates to 0.
The second solution is as follows; An interval between antennas of transceiver 3 is a Rayleigh interval, and an interval between antennas of transceiver 4 is also a Rayleigh interval. In this case, correlation of MIMO channels approximates to 0, thereby increasing channel capacity. For a 30 GHz frequency band, a Rayleigh distance required for 2 km transmission is 3 m. In this situation, an antenna array is difficult to achieve with a relatively large area.
In the process of implementing the present invention, the inventor finds at least the following problems in the prior art:
The first solution requires an extra relay base station to improve capacity of a microwave MIMO system, resulting in relatively high hardware costs, and a position selected for the relay base station is of vital importance, resulting in a difficulty in networking.
In the second solution, an interval between antennas of a transceiver is relatively large, the area of an antenna array is relatively large, and consequently the antenna array is difficult to achieve.